[mlir] Introduce IRDL dialect

This patch introduces the IRDL dialect, which allow users to represent
dynamic dialect definitions as an MLIR program.

The IRDL dialect defines operations, attributes, and types, using
attribute constraints. For example:

```
module {
  irdl.dialect @cmath {
    irdl.type @complex {
      %0 = irdl.is f32
      %1 = irdl.is f64
      %2 = irdl.any_of(%0, %1)
      irdl.parameters(%2)
    }

    irdl.operation @norm {
      %0 = irdl.any
      %1 = irdl.parametric @complex<%0>
      irdl.operands(%1)
      irdl.results(%0)
    }
}
```

This program will define a new `cmath.complex` type, which expects a single
parameter, which is either an `f32` or an `f64`. It also defines an
`cmath.norm` operation, which expects a single `cmath.complex` type as operand,
and returns a value of the underlying type. Note that like PDL (which IRDL is
heavily inspired from), both uses of `%0` are expected to be of the same attribute.

IRDL handles attributes and types with the same operations, and does this by always
wrapping types in a `TypeAttr`. This is to simplify the language.

Depends on D144690

Reviewed By: rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D144692